1. Field of the Invention
The present invention relates to cable modems and cable-modem systems, and particularly relates to a cable modem and a cable modem system which provide a connection to a LAN via a cable television transmission line.
2. Description of the Related Art
A cable television system (hereinafter referred to as a CATV system) distributes broadcast signals from a headend, which is a center of a CATV provider, to subscribers via trunk lines and branch lines, which are configured in a tree structure or in a star structure. The trunk lines and branch lines are used for transmission of broadband broadcast signals, and, thus, are implemented by using a coaxial cable or an optical fiber cable. This renders a CATV system a superior broadband transmission capacity.
In recent years, the Internet has made a significant market progress. Against this background, a superior broadband transmission capacity of a CATV system has been attracting attention, and efforts are underway to utilize CATV transmission lines as an access network to the Internet. A cable-modem system, in particular, is increasingly used in practice, connecting subscriber households to the Internet via a LAN interface utilizing CATV lines.
FIG. 1 is an illustrative drawing showing a configuration of a cable modem system. A description of the cable modem system will be given with reference to FIG. 1.
The cable modem system of FIG. 1 includes a CATV center 10, a CATV delivery facility 20, and a subscriber household 30. The CATV center 10 includes a center device 11, a headend device 12, photoelectric converters 13, a router 14, a server 15, personal computers 16, and bridges 17. The center device 11 is connected to an intra-CATV-center LAN at one end and to the headend device 12 at the other end. The headend device 12 transmits video signals and the like after demodulation and mixing of signals. The router 14 provides a connection to the Internet. The server 15 stores data therein to provide various services. The bridges 17 are used for connecting different LANs. The bridges 17 monitor packets, and reduces the number of packets in the LANs via a filtering function to dispose of unnecessary packets, thereby insuring transmission efficiency.
The CATV delivery facility 20 includes optical cables 21, photoelectric converters 22, coaxial cables 23, amplifiers 24, and tap-offs 25. Here, the tap-offs 25 are nodes or branching devices that connect between the CATV delivery facility 20 and the subscriber household 30.
The subscriber household 30 includes cable modems 31a through 31c, terminal devices 32a through 32d such as personal computers, and a bridge 17. The terminal device 32a, operating standalone, is connected to the CATV delivery facility 20 via the cable modem 31a. Further, a subscriber-household LAN 33 including the terminal devices 32b through 32d and the bridge 17 is connected to the CATV delivery facility 20 via the cable modems 31b and 31c. 
When a user accesses the Internet via the terminal device 32c, the terminal device 32c supplies packets to the cable modem 31b or 31c, where the packets are converted into RF signals to be supplied to the CATV delivery facility 20 via the tap-offs 25. The RF signals attenuate as they propagate through the CATV delivery facility 20, so that the amplifiers 24 are provided to boost signal levels. The RF signals are converted into optical signals by the photoelectric converters 22, and the optical signals are then sent via the optical cables 21.
The photoelectric converters 13 convert the optical signals received from the optical cables 21 into electrical signals, and supplies reconstructed RF signals to the center device 11 via the headend device 12. The center device 11 selects a frequency band that is used as an uplink band for the cable modems 31a through 31c, and converts the RF signals of this frequency band into packet signals. The reconstructed packets are sent to the Internet via the router 14. When packets are supplied from the Internet, on the other hand, transfer of the packets is carried out by following steps of the same procedure as described above in a reversed order. In this manner, the user can access the Internet by using the cable modems 31a through 31c. 
Unfortunately, there is a case in which a mistake in wire connections ends up causing the subscriber-household LAN 33 to create a closed loop, trapping packets inside. Such a loop may leads to a shutdown of the entire cable-modem system. In consideration of this, the bridge 17 is typically provided with a function to detect and sever a loop.
The bridge 17 in a conventional LAN system exchanges BPDU messages (packets) with other bridges by attaching its own MAC (media access control) address to these packets for the purpose of automatically detecting a loop. When a loop is detected, a bridge constituting part of the loop closes a relevant port thereof in order to sever the loop. This scheme is called a spanning tree scheme.
A cable-modem system is subject to standardization, and is required to serve as a bridge in compliance with standards set forth in the United States. A typical cable-modem system, therefore, adapts the spanning tree scheme in order to provide a function to detect and sever a loop in the same fashion as does a bridge of a conventional LAN system.
FIG. 2 is an illustrative drawing showing a logical configuration of a cable-modem system such as that shown in FIG. 1. With reference to FIG. 2, a description will be given with regard to a function to detect and sever a loop in a cable-modem system.
In FIG. 2, a center device 40 and cable modems 43a through 43e are each configured to serve as a bridge, and, for this purpose, are each provided with a filtering database 44. The filtering database 44 stores a definition as to whether to transit or dispose of a packet with respect to each MAC address. Such a definition is temporarily stored for a predetermined time period.
In the cable-modem system of FIG. 2, the center device 40 serves as a bridge BRIDGE0, and the cable modems 43a through 43n serve as bridges BRIDGE1 through BRIDGEn, respectively. The center device 40 has a port PORT0 thereof connected to the Internet via a router 41 and a port PORT1 thereof connected to a port PORT0 of each of the cable modems 43a through 43n. A port PORT1 of each of the cable modems 43a through 43n is connected to a terminal device 45 either directly or via a hub 46 and/or a bridge 47.
As described above, the center device 40 and the cable modems 43a through 43e are configured to serve as logically separate bridges, and are required to have a function to detect and sever a loop as such a loop may become a problem when implementing a LAN. Namely, each of the center device 40 and the cable modems 43a through 43e is so designed as to perform the spanning tree scheme.
Another configuration that can take care of the problem of a closed loop in a cable-modem system is to regulate communications between the cable modems 43a through 43e by requiring all the packets from the cable modems 43a through 43e to always make a transit at the center device 40. This can prevent formation of a closed loop.
FIG. 3 is an illustrative drawing showing another example of a logical configuration of a cable-modem system. The configuration of FIG. 3 has advantages in reduction of the load and conservation of resources.
The cable-modem system of FIG. 3 includes a center device 51 and cable modems 52a through 52n, all of which together constitute a logical bridge 50. The logical bridge 50 is equivalent to a bridge that has a port PORT0 on the side of the intra-CATV-center LAN and ports PORT1 through PORTn on the side of the subscriber-household LAN at the end of the cable modems 52a through 52n. 
FIG. 4 is a block diagram showing a configuration of the center device of FIG. 3.
As shown in FIG. 4, the center device 51 serving as part of the logical bridge 50 includes a LAN-interface unit 55, a bridge processing unit 56, and an RF-interface unit 57.
FIG. 5 is a block diagram showing a configuration of one of the cable modems shown in FIG. 3.
As shown in FIG. 5, the cable modem serving as part of the logical bridge 50 includes an RF-interface unit 57 and a LAN-interface unit 55. In the logical bridge 50 of FIG. 3, the cable modems 52a through 52n serve simply as repeaters, each of which transfers signals by boosting signal levels.
The filtering database 44 is provided only in the center device 51, and controls transit processing of the logical bridge 50. The bridge processing unit 56 consults the filtering database 44, and determines whether to dispose of a received packet or to transfer a received packet to the LAN-interface unit 55 or the RF-interface unit 57. When it is decided that a received packet is to be supplied to the RF-interface unit 57, an ID number of one of the cable modems 52a through 52n and an MAC address of a terminal device attached thereto are paired and stored in a cache table 53. Here, the one of-the cable modems 52a through 52n registered in the cache table 53 is a destination of the received packet.
By registering an ID number of a destination cable modem and an MAC address of a terminal device attached thereto in the cache table 53, an ID number of a cable modem can be searched for based on an MAC address of a packet destination when communication is effected between the cable modems 52a through 52n or between the center device 51 and one of the cable modems 52a through 52n. 
If the cache table 53 does not have an ID number and a MAC address stored therein, a packet is supplied to all the cable modems 52a through 52n. In this configuration shown in FIG. 3, therefore, no MAC address needs to be assigned to any one of the cable modems 52a through 52n. 
Related-art cable-modem systems are designed and implemented as described above.
In cable-mode systems having each cable modem serving as a bridge as those developed and subject to standardization in the United States, a mechanism for implementing the spanning-tree scheme is necessary in addition to the mechanism for the bridge function. This requires highly complex data processing, and, also, requires each cable modem to have an MAC address attached thereto. Another problem is a cost increase.
Further, packets transmitted from the cable modems are always required to make a transit at the center device in some, systems in order to regulate communication between the cable modems. Such a system has a drawback in that services options are rather limited.
A cable system that has a center device and cable modems together constituting a single logical bridge is not equipped with a function to automatically detect and sever a loop. If a loop is created in a subscriber-household LAM, therefore, an entire system may run a risk of having to be shutdown.
When a loop develops in the cable modem system of FIG. 3, for example, an entire system is shutdown when a broadcast MAC packet or a multicast MAC packet is supplied to the loop. This is because the bridge 47 is designed to transfer a received broadcast or multicast MAC packet to a port on the opposite side. When broadcast or multicast MAC packets are continuously transferred in the loop, therefore, this results in a sudden increase in traffic.
In order to provide a function to automatically detect and sever a loop, each one of the cable modems 52a through 52n needs to have an MAC address assigned thereto. Since the cable modems 52a through 52n merely serve as repeaters, no MAC addresses are normally assigned. When considering a current situation where there is an explosive increase in use of the Internet, it is desirable to detect and sever a loop without assigning to an MAC address to each of the cable modems 52a through 52n. 
Accordingly, there is a need for a cable modem and a cable-modem system which can detect and sever a loop without having MAC addresses assigned to cable modems.
Accordingly it is a general object of the present invention to provide a cable modem and a cable-modem system which can satisfy the need described above.
It is another and more specific object of the present invention to provide a cable modem and a cable-modem system which can detect and sever a loop without having MAC addresses assigned to cable modems.
In order to achieve the above needs according to the present invention, a system for communication via cable-television communication lines includes a center device which generates a loop-detection packet for detecting a loop that is a defect of a network configuration, and a cable modem which is situated between the center device and a subscriber end, and detects the loop by regarding a receipt of the loop-detection packet from the subscriber end as a detection of the loop.
The system described above can detect a loop created in a subscriber-household LAN. A principle underlying this loop detection is a premise that the loop-detection packet generated by the center device cannot be supplied to the cable modem from the subscriber end unless there is a loop at the subscriber end. Based on this principle, the present invention can detect and sever a loop.
According to another aspect of the present invention, a system for communication via cable-television communication lines includes a cable modem which is connected to a subscriber end, and counts broadcast or multicast packets supplied from the subscriber end to obtain a packet count, the cable modem detecting a loop that is a defect of a network configuration by regarding the packet count exceeding a predetermined number within a given constant interval as a detection of the loop.
The system described above can detect a loop created in a subscriber-household LAN. A principle underlying this loop detection is a premise that the number of broadcast or multicast packets supplied from the subscriber end within a given constant interval is not likely to exceed the predetermined number unless there is a loop at the subscriber end. Based on this principle, the present invention can detect and sever a loop.
According to another aspect of the present invention, a system for communication via cable-television communication lines includes a cable modem which is connected to a subscriber end, and stores source addresses of packets supplied from the subscriber end, the cable modem disposing of a packet supplied from the subscriber end if a destination address of the packet is one of the stored source addresses.
In the system as described above, the cable modem disposes of packets so as not to allow the packets to reach the center device when these packets are used for communication in a subscriber-household LAN, thereby insuring the privy of the communication.
According to another aspect of the present invention, a system for communication via cable-television communication lines includes a center device which stores an address of a source terminal device and an identifier of a source cable modem when receiving a packet from the source terminal device via the source cable modem, the address and the identifier being paired, and a cable modem, connected between the center device and a subscriber end, which stores in a memory an address of a source terminal device of a packet supplied from the subscriber end, and monitors addresses of packets supplied from the center device so as to dispose of a packet supplied from the center device and having a destination address not stored in the memory.
In the system described above, the cable modem disposes of the packet that is not supposed to be delivered to the terminal device connected to the cable modem. This insures that packets used for communication between a given terminal device and the Internet are not forwarded to another terminal device, thereby providing the privy of communication.
According to another aspect of the present invention, a system for communication via cable-television communication lines includes a center device, and a cable modem connected between the center device and a subscriber end, wherein both the center device and the cable modem dispose of a packet generated and supplied from a bridge if the packet has an address identifying the bridge.
The system as described above can prevent packets used for loop detection in the spanning-tree scheme from being exchanged between an intra-center LAN and a subscriber-household LAN.
In the spanning-tree scheme, the number of bridges between two ends needs to be fewer than seven in order to insure normal operations of the bridges. With such a requirement, a system having a change in the number of bridges in the subscriber-household LAN may encounter abnormal behavior of the bridges.
The system described above makes sure that the packets used for the spanning-tree scheme are not exchanged between the intra-center LAN and the subscriber-household LAN, so that the spanning-tree scheme should properly work within the confines of each LAN without being affected by other LAN systems.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.